In-Depth Notes on Population Genetics and Measuring Genetic Change

Lecture Outline

• Lecture 1 - Introduction to Population Genetics

  • What is Population Genetics (Pop Gen)

  • Gene Pools

  • Polymorphism

  • Genetic Variation

  • Microevolution

• Lecture 2 - Measuring Genetic Change

  • Allele and Genotype Frequencies

  • Five Forces of Evolutionary Change

  • Hardy-Weinberg Principle

  • Measure of Fitness

• Lecture 3 - Evolutionary Genetics

  • Natural Selection

  • Sexual Selection

  • Genetic Drift

  • Migration and Non-random Mating

Learning Outcomes

By the end of this lecture you should be able to:

• Calculate genotype and allelic frequencies, as well as heterozygosity, for a population

• Describe the five major forces of evolutionary change in natural populations

• Understand and apply the Hardy-Weinberg (H-W) Principle

• Explain how genetic drift causes changes in allele frequencies in small populations

Measuring Genetic Variation in Populations

Key Definitions

• Genotype Frequencies

  • Proportion of individuals with a specific genotype compared to the total

  • Example: In a population of 100 individuals with 40 AA genotypes, the frequency is $\frac{40}{100} = 0.4$

• Allelic Frequencies

  • Proportion of a specific allele compared to total alleles in a population

  • Total alleles in a diploid population = 2N (where N is the number of individuals)

• Heterozygosity

  • Observed frequency of heterozygotes compared to expected (H-W model)

Examples

1. Population of Poodle Moths:

   • 40 AA, 30 AB, 30 BB from 100 total
     data counting yields:

   • Genotype Frequencies:

     • Freq. AA = $\frac{40}{100} = 0.4$

     • Freq. AB = $\frac{30}{100} = 0.3$

     • Freq. BB = $\frac{30}{100} = 0.3$

2. Population of Quokkas:

   • 70 AA, 20 AB, 10 BB from 100 total
     data counting yields:

   • Genotype Frequencies:

     • Freq. AA = $\frac{70}{100} = 0.7$

     • Freq. AB = $\frac{20}{100} = 0.2$

     • Freq. BB = $\frac{10}{100} = 0.1$

Hardy-Weinberg Principle

Key Concepts

• Predicts allele and genotype frequencies will remain constant over generations in an ideal population

• Conditions for Hardy-Weinberg Equilibrium (HWE):

  1. No selection (equal reproductive success)

  2. No mutation

  3. No migration (no gene flow)

  4. Population is infinitely large (no genetic drift)

  5. Random mating occurs

Calculation

• Allele Frequency Equations:

  • For alleles A and a:

  • $p + q = 1$ (where p = frequency of A, q = frequency of a)

  • $p^2 + 2pq + q^2 = 1$ (where:

    • $p^2$ = frequency of AA

    • $q^2$ = frequency of aa

    • $2pq$ = frequency of Aa)

Heterozygote Frequency Calculation

• E.g., frequencies of A = 0.6 and B = 0.4:

  • Probability for AA: $0.6 x 0.6 = 0.36 o 36\%$

  • Probability for BB: $0.4 x 0.4 = 0.16 o 16\%$

  • Heterozygote AB: $2 x (0.6 x 0.4) = 0.48 o 48\%$

Forces of Genetic Change

• 1. Mating Systems

  • Random mating, inbreeding, assortative mating

• 2. Gene Flow

  • Migration between populations causing changes in allele frequencies

• 3. Mutation

  • Introduction of new alleles into a population

• 4. Natural Selection

  • Differential reproduction based on genotype fitness

• 5. Genetic Drift

  • Random changes in allele frequencies, stronger effects in smaller populations

  • Founder Effect: When a small group forms a new population

  • Bottleneck Effect: Population significantly reduced, leading to loss of genetic diversity

Genetic Caveats

• Effective Population Size: $N<em>E = \frac{4(N</em>m * N<em>f)}{N</em>m + N<em>f}$ (where Nm = number of males, N_f = number of females)

• Biological fitness represented as relative success, highest fitness denoted by 1.

• Dominant-recessive situations complicate selection due to recessive alleles being carried by heterozygotes.

Summary

• Five forces affecting allele frequencies (mating system, drift, mutation, selection, gene flow)

• HWE provides a baseline for testing if populations are evolving

• Observed genotype frequencies can be compared with expected frequencies using statistical tests, such as chi-square tests to find deviations from HWE.